Mesoscale Precipitation Discussion 0777
NWS Weather Prediction Center College Park MD
1013 PM EDT Wed Sep 05 2018

Areas affected...Central Rio Grande Valley into the Texas Hill
Country

Concerning...Heavy rainfall...Flash flooding possible

Valid 060213Z - 060713Z

Summary...Clusters of thunderstorms will develop across central
Texas and drift into the Texas Hill Country. Rainfall of 2-3
inches will have the potential to produce flash flooding,
especially in areas most sensitive due to terrain.

Discussion...Thunderstorms noted on Del Rio, TX WSR-88D will
continue to expand in coverage and drift across the region this
evening. Latest GOES-16 IR imagery shows cooling cloud tops
suggesting of intensifying convection, and this is reflected by
recent radar estimated rainfall rates of 1-1.5 inches per hour.

Latest RAP analysis of the thermodynamic environment shows a 2+
inch PWAT plume advecting onto the TX coast on ESE flow from the
surface up through 850mb, with moisture transport vectors directed
into central TX and the Balcones Escarpment. Although instability
is waning, sufficient MLCape of 500-1000 J/kg, 700mb confluence,
and subtle 300mb jet level diffluence will aid in enhancing lift
through the evening, and convection is forecast to increase in
coverage. Despite the modest lift, flash flooding will be possible
due to very slow storm motions. 0-6km mean wind on the 00Z DRT
sounding was just 2 knots from the south, and GFS corfidi vectors
are nearly anti-parallel to the mean flow suggesting almost
stationary movement of individual cells. With PWATs forecast to
continue to increase, nearly stationary cell movement could
produce 2-3" of rainfall in a very short period of time. This has
the potential to cause flash flooding, as 3-hr FFG values are
extremely low, less than 2 inches in some places, due to recent
heavy rainfall.

Weiss

ATTN...WFO...EWX...FWD...SJT...

ATTN...RFC...WGRFC...

LAT...LON   32439794 31989744 31229735 30579750 29919779
            29469830 29109897 28889967 28890035 29350107
            29910140 31220081 31579969 31689926 32179862